1. Field of the Invention
The present invention relates to a fuel cell. More specifically, the invention relates to a solid electrolyte-using or solid oxide fuel cell that can be advantageously put into practice in a system which generates electricity by arranging a fuel cell unit in the flame or near the flame so as to be exposed to the flame, or in a system which generates electricity relying upon a potential difference that takes place between an anode layer and a cathode layer by arranging the fuel cell unit in an atmosphere of a mixed fuel gas of a gaseous fuel and oxygen or an oxygen-containing gas. Particularly, the fuel cell of the invention features a high resistance against thermal shock, a high generation density per volume, is small in size, light in weight, can be designed permitting a high degree of freedom, can be excellently produced and at a low cost, and can be advantageously utilized as a generator in a variety of fields.
2. Description of Related Art
Fuel cells have heretofore been developed and put into practice, as power generation means which cause little pollution as substitutes for thermal power plants or as sources of electric energy for electric cars. The cells can replace engines burning gasoline or the like as a fuel. In recent years, in particular, extensive study has been aimed at providing fuel cells having high efficiency at low cost.
As is widely known, the fuel cells have been provided in a variety of generation types. Among them, a fuel cell of the type of using a solid electrolyte, i.e., a solid oxide fuel cell (SOFC) is expected to offer the highest generation efficiency, further featuring a long life and a low cost, and is drawing attention in various fields.
According to one example, the solid electrolytic fuel cell uses a fired product of stabilized zirconia to which yttria (Y2O3) has been added as a solid electrolytic layer of the oxygen-ion conduction type. A cathode layer is formed on one surface of the solid electrolytic layer, an anode layer is formed on the opposite surface thereof, oxygen or an oxygen-containing gas is supplied to the side of the cathode layer, and a fuel gas such as methane is supplied to the anode layer. A fuel cell unit comprising the solid electrolytic layer, the anode layer and the cathode layer is contained in a chamber to complete a fuel cell.
In this fuel cell, oxygen (O2) supplied to the cathode layer is ionized into oxygen ions (O2−) in a boundary between the cathode layer and the solid electrolytic layer, the oxygen ions are conducted to the anode layer through the solid electrolytic layer, reacted with, for example, a methane (CH4) gas that is supplied to the anode layer and, finally, form water (H2O) and carbon dioxide (CO2). In this reaction, the oxygen ions release electrons producing a potential difference between the cathode layer and the anode layer. By attaching lead wires to the cathode layer and to the anode layer, therefore, the electrons in the anode layer flow to the side of the cathode layer through the lead wire generating electricity as a fuel cell. However, the fuel cell of this so-called separate chamber type is, usually, operated at a temperature which is as high as about 1000° C. Besides, the cathode layer side is exposed to an oxidizing atmosphere and the anode layer side is exposed to a reducing atmosphere making it difficult to stably use the fuel cell unit over extended periods of time and, hence, it lacks durability.
SCIENCE, Vol. 288 (2000), pp. 2031-2033, as schematically shown in FIG. 1, suggests a fuel cell of the so-called single chamber type containing, in a chamber 110, a fuel cell unit 106 that has a cathode layer 102 and an anode layer 104 formed on both surfaces of a solid electrolytic layer 100. In this fuel cell, a mixed fuel gas which is a mixture of a methane gas and oxygen is introduced into the chamber 110 through a conduit 110a, whereby the fuel cell unit 106 generates an electromotive force due to the action of the mixed fuel gas. The exhaust gas, after use, is exhausted out of the chamber through a conduit 110b. In the case of this fuel cell, however, only one fuel cell unit is contained in the chamber, and the voltage that is taken out is not high enough for practical use.
In the above fuel cell, the fuel cell unit is contained in a chamber. There has further been proposed a fuel cell of the so-called direct flame type which generates electricity by arranging the solid oxide fuel cell unit in the flame or near the flame to maintain the fuel cell unit at its operation temperature by the heat of the flame, in an attempt to simplify the structure, to decrease the size and weight and to decrease the cost. In the fuel cell of this kind, in particular, the electromotive time can be shortened owing to the direct use of the flame.
As an example of the solid oxide fuel cell which utilizes the flame, Japanese Unexamined Patent Publication (Kokai) No. 6-196176 (JP-A-6-196176) discloses a fuel cell provided with a tubular solid oxide fuel cell. FIG. 2 illustrates an example of the combustion device therefor, wherein a fuel cell unit 203 comprises a zirconia solid electrolytic pipe 212a, an anode layer 222 which is a fuel electrode formed on the outer side of the pipe 212a, and a cathode layer (not shown) which is an air electrode formed on the inner side of the pipe 212a. The solid oxide fuel cell unit 203 is so installed that the anode layer 222 is exposed to a reducing flame portion 223 of a flame 202 produced by the combustion device 201 to which the fuel gas is supplied. Thus, by installing the fuel cell unit 203 in the flame 202, electricity can be generated by utilizing, as a fuel, radical components existing in the reducing flame.
In the present invention, as will be described later in detail, the structure for supporting the fuel cell plays an important role. For easy comprehension of a novel support structure of the present invention, a representative example of a conventional electrode-supported fuel cell will be summarized below.
A structure of the flat plate type has been studied as a step toward putting the solid oxide fuel cell into a practical use. Namely, the cell has a three-layer structure of cathode/solid electrolyte/anode, and electricity is collected from the cell and the gas is separated by an interconnecting member (separator) having such a channel structure that the air flows toward the cathode side and the fuel flows toward the anode side. Therefore, the interconnecting member must be electron conductive and gas tight. These structures have been disclosed in, for example, Ceramic, 30 (1995), No. 4, pp. 329-332, and Journal of the Electrochemical Society, 150(9), A1188A-A1201 (2003).
On the other hand, the fuel cell unit, too, must have mechanical strength from the standpoint of the handling thereof. So far, in general, a ceramic substrate of zirconia or ceria is prepared to serve as a solid electrolytic layer, and a cathode layer and an anode layer are formed on both surfaces thereof to prepare a cell. The solid electrolytic layer requires gas tightness for preventing the mixing of the fuel and the air, while the cathode layer and the anode layer must have porosities of not smaller than a predetermined value such that the reaction species can be diffused to a sufficient degree. Therefore, the above production method is desirable even from the standpoint of realizing a difference in the densities thereof (first, the electrolyte is fired at a high temperature and, after an electrode material is applied thereon, is additionally fired at a lower temperature to form an electrode layer of a low density on the electrolytic layer of a high density).
Forming the electrolytic layer in a decreased thickness is effective in decreasing the internal resistance of the fuel cell unit, and it has been studied to form the electrolyte at a thickness of several tens of microns. In this case, a sufficient degree of strength is not obtained by the electrolytic layer as a matter of course and, hence, at least one of the electrode layer must be thickly formed to maintain a strength greater than a predetermined level. For this reason, there has been employed a structure for supporting either the anode (fuel electrode) or the cathode (air electrode). Joep Hujismans, in the Fifth European Solid Oxide Fuel Cell Forum, Proceedings Vol. 1, 2002 July, pp. 116-117 related to Proceedings of an International Academy related to SOFC, 2002, reports and discusses, in a session, whether the electrolyte or the electrode should be supported. A first theory briefly describes the background. Further, the Journal of the Electrochemical Society, 151(8), A1128-A1133 (2004) introduces an experiment on an anode-supported cell, and the Ninth Meeting of Reading Study Papers on SOFC, Proceedings, December, 2000, SOFC Academy, pp. 37-40 introduces an experiment on a cathode-supported cell.
As described above, the portion supporting the cell supports either the electrolytic layer or the electrode layer. In experiments, however, a metal mesh is used in many cases not for the purpose of support but for the purpose of electric connection. A single cell is evaluated on an experimental level not relying on a large-scale structure that uses the above interconnecting member but, usually, by drawing wires from the electrodes in a simple manner. Though this has not been closely described in recent literature, attempts have been made to connect lead wires such as of silver, gold or platinum to the mesh and bury them in the electrode material layer. J. Electrochem. Soc., Vol. 145, No. 5, 1998 May, pp. 1696-1701 discloses an example in which a platinum mesh is connected to the electrode layer by using an anode material (Ni-cermet), and shows in cross section the structure of part of the layers. Materials Transactions, JIM, Vol. 41, No. 12(2000), pp. 1621-1625 teaches a similar connection in which the electrode has a diameter of 13 mm while the platinum mesh that is used has a size of 2 mm square. The metal lead can be connected only to the electrode layer without using the mesh but has a very low mounting strength. Therefore, a mesh is used in many cases. This is done when the single cell is evaluated on an experimental level. On a practical level, however, an interconnecting member is used.